Deft

Deft is an Emacs mode for quickly browsing, filtering, and editing directories of plain text notes. It is inspired by the OS X app Notational Velocity (see also nvAlt). Deft is designed for increased productivity when writing and taking notes, by making it fast and simple to find the right file at the right time
and by automating many of the usual tasks such as creating new files and saving files

Deft notes can be stored in the cloud using something like Dropbox. Simply configure Deft to use a directory that is being synced:

(setq deft-directory "~/Dropbox/notes/")

If you use the online service SimpleNote (e.g. to read and capture notes on an iOS device) you can combine this with Dropbox sync and Deft. SimpleNote looks for .txt files and uses the file name as the note title, so configure Deft to do the same: